­­This is an AcrosticKyrielle, inspired by recent research which found that exercise-induced endorphin release depends on the intensity of the exercise and that High-Intensity Interval Training (HIIT) leads to a larger release of endorphins than regular aerobic exercise, which can ultimately result in negative feelings.

When you exercise, your body releases chemicals called endorphins. These endorphins interact with opioid receptors in your brain, thereby reducing the perception of pain and acting as a coping mechanism that may promote habitual exercise. Participants in this study were injected with a radioactive compound that binds itself to the brain’s opioid receptors, whose activity was then measured using a PET (Positron Emission Tomography) scanner to determine the rate of endorphin production. The same participants were measured after three different activities: a 60-minute aerobic, moderate-intensity session; a HIIT session; and a period of rest. The study found that whilst both sets of exercise resulted in the release of endorphins, the number of endorphins released after a HIIT workout were significantly higher.

The higher endorphin release by the HIIT workout might explain how the high physical and emotional stress that is produced by such a workout is alleviated. However, an excess release of endorphins can also cause the brain to be flooded with “fight-or-flight” hormones, leading to negative feelings of rage or anxiety. The researchers in this investigation found that such negative feelings were observed in some of the participants following a HIIT workout, concluding that whilst these endorphins are needed to manage the emotionally and physically demanding challenges of HIIT, the negative feelings that they can result in might actually discourage further exercise.